Electromagnetic CompatibilityVesa Linja-aho – 2012-07-19
Me
Senior Lecturer in Automotive Electronics at Metropolia UAS
Background: M.Sc. in Electromagnetics and Circuit Theory, Helsinki University of Technology
Worked as researcher, lecturer and journalist. Interested in Open Educational Resources and
Electric Work Safety.
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Metropolia UAS
In Finland, we have two kinds of institutions of higher education University: Bachelor (3 yr), Master (2 yr), PhD
(3-4 yr) University of Applied Sciences (UAS): Bachelor
(4 yr) and Master (2 yr) Metropolia is the largest UAS in Finland Automotive engineering education:
Formula Student –team Electric Raceabout (ERA)
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Studying in Finland
Metropolia unfortunately does not have automotive engineering degree programmes in English, unfortunately.
But we have other interesting programmes: http://www.metropolia.fi/en/
In Finland, we do not have tuition fees (some universities charge a tuition fee from non-EEA citizens, but not Metropolia).
The application time is at spring.
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EMC = Electromagnetic Compatibility
All electric and electronic devices should be designed so that they will accept any normal interference won’t interfere other devices.
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Everyday Examples?
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Everyday Examples
The ”GSM song” FM radio crackling when your neighbor drills a
hole on his wall. FM radio clicking when fluorescent tubes start.
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Not-so-everyday Examples
Electric wheelchair turns in the water when a police officer pushes talk button in the police boat radio.
Piezoelectric cigarette lighter opens barrier at a parking lot.
Starting a welding transformer causes the central computer to crash (in another building).
The roof and central locking system react to cellphone.
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Electromagnetic Interference (EMI)
Natural interference: cosmic radiation and thunderstorm.
Technical interference: Electrostatic discharge (ESD) Digital circuits Changes in the mains voltage Wireless communication
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Coupling Mechanisms
Conducting Capacitive coupling (via electric field) Inductive coupling (via magnetic field) Electromagnetic waves
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Coupling Mechanisms
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How to Fight EMI?
Prevent the emergence of interference Cut the path of propagation Improve the tolerance for the interference
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Countermeasures in Practice
Layout design and position of the wires Symmetrical transmission Filtering Using digital electronics Using optical transmission
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Brief Physical Background
An electric charge creates an electric field. Electric current or changing electric field creates
a magnetic field. Changing magnetix field induces a voltage into a
wire.
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Capacitive coupling = Coupling via Electric FieldCountermeasures Metallic enclosing Increasing the distance between wires Positioning the wires near the ground plane Decreasing the impedance level
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Inductive Coupling = Coupling via Magnetic FieldCountermeasures Layout design Decreasing the impedance level
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EMC is not a Separate Matter
The risky way: design the product and fix the EMC issues afterwards.
The safe way: keep EMC in mind during the whole design process
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Common Mode vs. Differential Signal
Common mode signal together with the ground plane causes a large loop between circuits.
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Three-level Protection
Layout design Interface filtering Metallic enclosing
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Do we need protection?
In simple non-critical devices, the layout design is often enough. Especially, if there are no cables to/from the
device. About 90 % of post-design EMC-problems are
caused by poor layout design!
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Good layout design
Split the system in parts Think the ground plane as a large current
conducting path. Choose grounding points well and minimize the
grounding impedance. Remember that every conducting part can carry
interfering currents!
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Splitting the system in parts
Decide which parts are critical vs. non-critical. Place the parts which are neither sensitive nor
sources for interference, into separate locations even on their own circuit boards.
For example, linear power supplies, non-clocked logic circuits and power amplifiers are usually immune to interference.
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Grounding
The ground plane is non-ideal. The correct grounding style depends on the circuit.
Single-point grounding is common in switching-mode power supplies. It prevents interfering voltages caused by currents through common impedances. But: on large frequencies, the wires act as
transmission lines! Multi-point grounding. Works well on large
frequencies. Each circuit has its own ground, and the grounds are interconnected with short wires.
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Rule of thumb for grounding
< 1 MHz: Single-point grounding > 10 MHz: Multi-point grounding
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Using a ground plane
If you use a multilayer PCB, using a ground plane is possible and recommended.
With RF circuits and fast digital circuits using the ground plane is practically mandatory.
What is the purpose of the large ground plane?
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The purpose of the ground plane
The main purpose is to provide low grounding impedance.
The secondary purpose is to act as a shield.
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Crosstalk
Two wires on the PCB are – unfortunately – connected to each other capacitively and inductively.
In practice, there is no cross-talk if the distance between wires is larger than 1 cm.
Dropping the impedance and using the ground plane will reduce crosstalk.
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Design Checklist
Avoid long wires on the PCB board. Sensitive and interfering components should not
be placed near each other. Do not place sensitive parts near the edges of
the ground plane. Split the circuit in parts very carefully.
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Interfaces and filtering
Most devices are interconnected to other devices via a cable.
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Ferrite chokes
Very common in data cables. Adds series inductance to cable. Effective on frequencies 1-1000 MHz
(approximately). Disadvantage: relatively low attenuation (10-20
dB). Advantage: easy to add afterwards. Ferrite choke attenuates also fast transients
caused by ESD.
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Mains filtering
Example: an electric shock from computer chassis.
Maximum leak current 0,5 mA. (EN 60601-1-1)
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Cables
Coaxial cable Twisted pair Shielded twisted pair
The shield should be connected evenly – avoid the pig tail mistake!
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Switches
Switches cause disturbance in two ways: Arcing Bouncing
Use RC snubber
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Relays
RC snubber + protective diode
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Electric motors
Motors cause strong magnetic fields. If the motor has commutator or brushes, the
arcing causes wide-spectrum RF interference. Inverter-driven permanent magnet machines are
more EMC-safe.
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ESD protection
Have you ever destroyed anything with ESD. What precautions you should take when handling
and installing an expensive graphics card to your computer?
MOSFET components are the most sensitive part to ESD.
The main method for ESD protection is protective diodes.
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Legislation and Standards
Harmonized by European Union EMC-directive 2004/108/EY Radio and telecommunications equipment
1999/5/EC Automotive EMC-directive 2004/104/EC. European standards and national laws.
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There are Plenty of Standards
Many device types have their own standard. If there is no standard available, the general
standard is to be applied.
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EMC Testing
EMI immunity EMI emissions
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EMC Testing Case Example: Electric Raceabout
Name surname 42
www.metropolia.fi/en/www.facebook.com/[email protected]
Thank you!